Multijunction solar cell assembly for space applications

ABSTRACT

A multijunction solar cell assembly and its method of manufacture including first and second discrete and different semiconductor solar cells which are electrically interconnected to form a four or five junction solar cell assembly.

REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 16/543,833 filed Aug. 19, 2019, which was a divisionalapplication of U.S. patent application Ser. No. 15/250,673 filed Aug.29, 2016, now U.S. Pat. No. 10,403,778, which claimed the benefit ofU.S. Provisional Patent Application Ser. Nos. 62/243,239 filed Oct. 19,2015 and 62/288,181 filed Jan. 28, 2016.

The present application is related to U.S. patent application Ser. No.15/203,975 filed Jul. 7, 2016, and U.S. patent application Ser. No.15/250,643 filed Aug. 29, 2016, now U.S. Pat. No. 10,270,000.

This application is also related to U.S. patent application Ser. No.15/210,532 filed Jul. 14, 2016, and U.S. Ser. No. 15/213,594 filed Jul.19, 2016, now U.S. Pat. No. 10,361,330.

This application is also related to U.S. patent application Ser. No.14/660,092 filed Mar. 17, 2015, which is a division of U.S. patentapplication Ser. No. 12/716,814 filed Mar. 3, 2010, now U.S. Pat. No.9,018,521; which was a continuation in part of U.S. patent applicationSer. No. 12/337,043 filed Dec. 17, 2008.

This application is also related to co-pending U.S. patent applicationSer. No. 13/872,663 filed Apr. 29, 2012, which was also acontinuation-in-part of Application Ser. No. 12/337,043, filed Dec. 17,2008

This application is also related to co-pending U.S. patent applicationSer. Nos. 14/828,197 and 14/828,206 filed Aug. 17, 2015.

All of the above related applications are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to solar cells and the fabrication ofsolar cells, and more particularly the design and specification of amultijunction solar cell using electrically coupled but spatiallyseparated semiconductor bodies based on III-V semiconductor compounds.

Description of the Related Art

Solar power from photovoltaic cells, also called solar cells, has beenpredominantly provided by silicon semiconductor technology. In the pastseveral years, however, high-volume manufacturing of III-V compoundsemiconductor multijunction solar cells for space applications hasaccelerated the development of such technology not only for use in spacebut also for terrestrial solar power applications. Compared to silicon,III-V compound semiconductor multijunction devices have greater energyconversion efficiencies and generally more radiation resistance,although they tend to be more complex to manufacture. Typical commercialIII-V compound semiconductor multijunction solar cells have energyefficiencies that exceed 27% under one sun, air mass 0 (AM0),illumination, whereas even the most efficient silicon technologiesgenerally reach only about 18% efficiency under comparable conditions.Under high solar concentration (e.g., 500×), commercially availableIII-V compound semiconductor multijunction solar cells in terrestrialapplications (at AM1.5D) have energy efficiencies that exceed 37%. Thehigher conversion efficiency of III-V compound semiconductor solar cellscompared to silicon solar cells is in part based on the ability toachieve spectral splitting of the incident radiation through the use ofa plurality of photovoltaic regions with different band gap energies,and accumulating the current from each of the regions.

In satellite and other space related applications, the size, mass andcost of a satellite power system are dependent on the power and energyconversion efficiency of the solar cells used. Putting it another way,the size of the payload and the availability of on-board services areproportional to the amount of power provided. Thus, as payloads becomemore sophisticated, the power-to-weight ratio of a solar cell becomesincreasingly more important, and there is increasing interest in lighterweight, “thin film” type solar cells having both high efficiency and lowmass.

The efficiency of energy conversion, which converts solar energy (orphotons) to electrical energy, depends on various factors such as thedesign of solar cell structures, the choice of semiconductor materials,and the thickness of each cell. In short, the energy conversionefficiency for each solar cell is dependent on the optimum utilizationof the available sunlight across the solar spectrum. As such, thecharacteristic of sunlight absorption in semiconductor material, alsoknown as photovoltaic properties, is critical to determine the mostefficient semiconductor to achieve the optimum energy conversion.

Typical III-V compound semiconductor solar cells are fabricated on asemiconductor wafer in vertical, multijunction structures or stackedsequence of solar subcells, each subcell formed with appropriatesemiconductor layers and including a p-n photoactive junction. Eachsubcell is designed to convert photons over different spectral orwavelength bands to electrical current. After the sunlight impinges onthe front of the solar cell, and photons pass through the subcells, thephotons in a wavelength band that are not absorbed and converted toelectrical energy in the region of one subcell propagate to the nextsubcell, where such photons are intended to be captured and converted toelectrical energy, assuming the downstream subcell is designed for thephoton's particular wavelength or energy band.

The individual solar cells or wafers are then disposed in horizontalarrays, with the individual solar cells connected together in anelectrical series and/or parallel circuit. The shape and structure of anarray, as well as the number of cells it contains, are determined inpart by the desired output voltage and current.

The electrical characteristics of a solar cell, such as the shortcircuit current density (J_(sc)), the open circuit voltage (V_(oc)), andthe fill factor, are affected by such factors as the number of subcells,the thickness of each subcell, the composition and doping of each activelayer in a subcell, and the consequential band structure, electronenergy levels, conduction, and absorption of each subcell, as well asits exposure to radiation in the ambient environment over time. Theoverall power output and conversion efficiency of the solar cell arethereby affected in different and often unpredictable ways. Such factorsalso vary over time (i.e. during the operational life of the system).

Accordingly, it is evident that the consideration of any one designparameter or variable, such as the amount of a particular constituentelement in a layer, or the band gap of that layer, affects each of theelectrical characteristics in a different way, sometimes in oppositedirections, and such changes does not predictably lead to an increase inpower out or solar cell efficiency. Stated another way, focus on any onesuch parameter in the design of a multijunction solar cell is not aviable calculus since each variable standing alone is NOT a simple“result effective” variable that can be automatically adjusted by thoseskilled in the art confronted with complex design specifications andpractical operational considerations in order to achieve greater poweroutput or a related design objective.

Another parameter of consideration taught by the present disclosure isthe difference between the band gap and the open circuit voltage, or(E_(g)/q−V_(oc)), of a particular active layer, and such parameter mayvary depending on subcell layer thicknesses, doping, the composition ofadjacent layers (such as tunnel diodes), and even the specific waferbeing examined from a set of wafers processed on a single supportingplatter in a reactor run.

One of the important mechanical or structural considerations in thechoice of semiconductor layers for a solar cell is the desirability ofthe adjacent layers of semiconductor materials in the solar cell, i.e.each layer of crystalline semiconductor material that is deposited andgrown to form a solar subcell, have similar crystal lattice constants orparameters. The present application is directed to solar cells withseveral substantially lattice matched subcells, and in a particularembodiment to a five junction (5J) solar cell using electrically coupledbut spatially separated four junction (4J) semiconductor bodies based onIII-V semiconductor compounds.

SUMMARY OF THE DISCLOSURE Objects of the Disclosure

It is an object of the present disclosure to provide increasedphotoconversion efficiency in a multijunction solar cell for spaceapplications over the operational life of the photovoltaic power system.

It is another object of the present disclosure to provide in amultijunction solar cell in which the selection of the composition ofthe subcells and their band gaps maximizes the efficiency of the solarcell at a predetermined high temperature (in the range of 40 to 70degrees Centigrade) in deployment in space at AM0 at a predeterminedtime after the initial deployment, such time being at least one, five,ten, fifteen, or twenty years.

It is another object of the present invention to provide a four junctionsolar cell in which the average band gap of all four cells is greaterthan 1.44 eV.

It is another object of the present invention to provide two differentlattice matched four junction solar cell subassemblies or bodies inwhich the current through the bottom subcell of each subassembly isintentionally designed to be substantially greater than current throughthe top three subcells when measured at the “beginning-of-life” or timeof initial deployment.

It is another object of the present invention to provide a five-junction(5J) solar assembly assembled from two different four-junction (4J)solar cell subassemblies.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingobjects.

Features of the Invention

Briefly, and in general terms, the present disclosure describes solarcells that include a solar cell assembly of two or more solar cellsubassemblies, each of which includes a respective monolithicsemiconductor body composed of a tandem stack of solar subcells, wherethe subassemblies are interconnected electrically to one another.

All ranges of numerical parameters set forth in this disclosure are tobe understood to encompass any and all subranges or “intermediategeneralizations” subsumed herein. For example, a stated range of “1.0 to2.0 eV” for a band gap value should be considered to include any and allsubranges beginning with a minimum value of 1.0 eV or more and endingwith a maximum value of 2.0 eV or less, e.g., 1.0 to 1.2, or 1.3 to 1.4,or 1.5 to 1.9 eV.

As described in greater detail, the inventors of the present applicationhave discovered that interconnecting two or more spatially splitmultijunction solar cell subassemblies can be advantageous. The spatialsplit can be provided for multiple solar cell subassemblies assembled ona single support or, and substrate coupled together electrically.

One advantage of interconnecting two or more spatially splitmulti-junction solar cell subassemblies is that such an arrangement canallow accumulation of the current of different subcell arrangementsfabricated in different semiconductor bodies.

Further, selection of relatively high band gap semiconductor materialsfor the top subcells can provide for increased photoconversionefficiency in a multijunction solar cell for outer space or otherapplications over the operational life of the photovoltaic power system.For example, increased photoconversion efficiency at a predeterminedtime after initial deployment of the solar cell can be achieved.

The subcells are configured so that the current density of the upperfirst subcell and the second subcell have a substantially equalpredetermined first value, and the current density of the bottom subcellis at least twice that of the predetermined first value.

Briefly, and in general terms, the present disclosure provides a fivejunction solar cell assembly comprising including a terminal of firstpolarity and a terminal of second polarity comprising: a firstsemiconductor body including a tandem vertical stack of at least a firstupper, a second and a bottom solar subcells; and a second semiconductorbody disposed adjacent and parallel to the first semiconductor body andincluding a tandem vertical stack of at least a first upper, a secondand a bottom solar subcells substantially identical to that of the firstsemiconductor body, the first upper subcell of the first and secondsemiconductor bodies having a top contact connected to the terminal offirst polarity, the third bottom subcell of the second semiconductorbody having a bottom contact connected to the terminal of secondpolarity; wherein the third subcell of the first semiconductor body isconnected in a series electrical circuit with the third subcell of thesecond semiconductor body so that the interconnection of subcells of thefirst and second semiconductor bodies forms at least a four junctionsolar cell; and wherein the sequence of layers in the first and thesecond semiconductor bodies are different.

In some embodiments, the upper first subcell of the first and secondsemiconductor bodies is composed of indium gallium phosphide (InGaP);the second solar subcell of the first and second semiconductor bodiesdisposed adjacent to and lattice matched to said upper first subcell,the second solar subcell composed of aluminum gallium arsenide (AlGaAs)or indium gallium arsenide phosphide (InGaAsP), and the third subcell isthe bottom subcell of each of the semiconductor bodies and is latticematched to said second subcell and is composed of germanium (Ge).

In some embodiments, the upper first subcell of the first semiconductorbody is composed of aluminium indium gallium phosphide (AlInGaP); thesecond solar subcell of the first semiconductor body is disposedadjacent to and lattice matched to said upper first subcell, and iscomposed of aluminum gallium arsenide (AlGaAs); the third subcell isdisposed adjacent to and lattice matched to said second subcell and iscomposed of gallium arsenide (GaAs), and the bottom subcell of the firstand second semiconductor body is lattice matched to said second subcelland is composed of germanium (Ge).

In some embodiments, the first semiconductor body further comprises afirst highly doped lateral conduction layer disposed adjacent to andbeneath the second solar subcell.

In some embodiments, the second semiconductor body further comprises asecond highly doped lateral conduction layer disposed adjacent to andbeneath the second solar subcell, and a blocking p-n diode or insulatinglayer disposed adjacent to and beneath the second highly doped lateralconduction layer, and a third highly doped lateral conduction layerdisposed adjacent to and beneath the blocking p-n diode or insulatinglayer.

In some embodiments, the short circuit density (J_(sc)) of each of thebottom subcells is at least twice that of the first and second subcells.

In some embodiments, the short circuit current density (J_(sc)) of thefirst and second subcells are each approximately 17 mA/cm², and theshort circuit current density (J_(sc)) of each of the bottom subcells isapproximately 34 mA/cm².

In some embodiments, the short circuit current density (J_(sc)) of thefirst, second and third middle subcells are each approximately 11mA/cm².

In some embodiments, the short circuit current density (J_(sc)) of eachof the bottom subcells is approximately 22.6 mA/cm².

In some embodiments, at least the base of at least one of the first,second or third solar subcells has a graded doping.

In some embodiments, there further comprises a third middle solarsubcell composed of gallium arsenide (GaAs) disposed adjacent to andbeneath the second solar subcell, and above the bottom solar subcell.

In some embodiments, there further comprises a first conductiveinterconnect extending between the contact layer of the first uppersubcell of the first semiconductor body to the contact layer of thefirst upper subcell of the second semiconductor body.

In some embodiments, there further comprises a second conductiveinterconnect extending between the bottom contact layer of the thirdsubcell of the first semiconductor body to the bottom contact layer ofthe third subcell of the second semiconductor body.

In some embodiments, there further comprises a third conductiveinterconnect extending between the bottom contact layer of the bottomsubcell of the first semiconductor body to the top contact layer of thebottom subcell of the second semiconductor body.

In some embodiments, there further comprises a third semiconductor bodydisposed adjacent to the second semiconductor body and including atandem vertical stack of at least a first upper, a second, third and afourth bottom solar subcells, the first upper subcell having a topcontact connected to the terminal of first polarity, the fourth bottomsubcell having a bottom contact connected to the terminal of a secondpolarity; wherein the top contact of the first upper subcells of thefirst, second and third semiconductor bodies are connected, and thefourth subcell of the first semiconductor body is connected in a serieselectrical circuit with the fourth subcell of the second semiconductorbody, which in turn is connected in a series electrical circuit with thefourth subcell of the third semiconductor body.

In some embodiments, the respective selection of the composition, bandgaps, open circuit voltage, and short circuit current of each of thesubcells maximizes the efficiency of the assembly (i) at hightemperature (in the range of 40 to 100 degrees Centigrade) in deploymentin space at a predetermined time after the initial deployment (referredto as the beginning of life or BOL), such predetermined time beingreferred to as the end-of-life (EOL), wherein such predetermined time isin the range of one to twenty-five years; or (ii) at low temperature (inthe range of −150 to −100 degrees Centigrade), and low solar radiationintensity less than 0.1 suns, in deployment in space at a predeterminedtime after the initial deployment (referred to as the beginning of lifeor BOL), such predetermined time being referred to as the end-of-life(EOL), wherein such predetermined time is in the range of one totwenty-five years.

In some embodiments, one or more of the subcells have a base regionhaving a gradation in doping that increases exponentially from a valuein the range of 1×10¹⁵ to 1×10¹⁸ free carriers per cubic centimeteradjacent the p-n junction to a value in the range of 1×10¹⁶ to 4×10¹⁸free carriers per cubic centimeter adjacent to the adjoining layer atthe rear of the base, and an emitter region having a gradation in dopingthat decreases from a value in the range of approximately 5×10¹⁸ to1×10¹⁷ free carriers per cubic centimeter in the region immediatelyadjacent the adjoining layer to a value in the range of 5×10¹⁵ to 1×10¹⁸free carriers per cubic centimeter in the region adjacent to the p-njunction.

In another aspect, the present disclosure provides a method of forming asolar cell assembly including a terminal of first polarity and aterminal of second polarity comprising: forming first and secondsemiconductor bodies, each including an identical tandem vertical stackof at least an upper first, a second and a third solar subcells, and abottom solar subcell; mounting the second semiconductor body adjacent tothe first semiconductor body; providing a bottom contact on the bottomsubcell of the second semiconductor body; connecting the bottom contacton the bottom subcell of the second semiconductor body to the terminalof second polarity; connecting the third subcell of the firstsemiconductor body in a series electrical circuit with the third subcellof the second semiconductor body so that at least a four junction solarcell is formed by the assembly; and providing a top electric contact onthe upper first subcell of the first and second semiconductor bodies andelectrically connecting each of the top electrical contacts to theterminal of first polarity.

In some embodiments, the average band gap of all four subcells (i.e.,the sum of the four band gaps of each subcell divided by 4) in eachsemiconductor body is greater than 1.44 eV, and the fourth subcell iscomprised of a direct or indirect band gap material such that the lowestdirect band gap of the material is greater than 0.75 eV.

In some implementations, the average band gap of all of the subcells isgreater than 1.44 eV. In some instances, the band gap of the first uppersubcell is in the range of 2.0 to 2.20 eV, the band gap of the secondsubcell is in the range of 1.65 to 1.8 eV, the third subcell has a bandgap of approximately 1.41 eV, and the band gap of the bottom subcell isin the range of 0.6 to 0.8 eV. Other implementations may have differentband gap ranges.

In some implementations, the first semiconductor body further includesone or more of the following features. For example, there may be a firsthighly doped lateral conduction layer disposed adjacent to the fourthsolar subcell. The first semiconductor body also can include a blockingp-n diode or insulating layer disposed adjacent to and above the highlydoped lateral conduction layer. The first semiconductor body may furtherinclude a second highly doped lateral conduction layer disposed adjacentto and above the blocking p-n diode or insulating layer. A metamorphiclayer can be disposed adjacent to and above the second highly dopedlateral conduction layer.

Some implementations can include additional solar subcells in one ormore of the semiconductor bodies.

The solar cell subassembly can further include a plurality of openingsin the first semiconductor body, each of the openings extending from atop surface of the first semiconductor body to a different respectivecontact layer in the first semiconductor body. Thus, for example, afirst opening in the first semiconductor body can extend from the topsurface of the semiconductor body to the first lateral conduction layer.A metallic contact pad can be disposed on the lateral conduction layer.A second opening in the first semiconductor body can extend from the topsurface of the semiconductor body to the contact back metal later of thebottom subcell.

In some implementations, the short circuit density (J_(sc)/cm²) of the(Al)InGaP first upper subcell is approximately 12 mA/cm². The shortcircuit density (J_(sc)/cm²) of the first upper subcell may have anothervalue for different implementations.

In another aspect, a solar cell assembly includes a terminal of firstpolarity and a terminal of second polarity. The solar cell assemblyincludes a first semiconductor body including a tandem vertical stack ofat least a first upper, a second, a third and a fourth solar subcell,the first upper subcell having a top contact connected to the terminalof first polarity. The solar cell assembly further includes a secondsemiconductor body disposed adjacent to the first semiconductor bodycomposed of different layers than the first semiconductor body andincluding a tandem vertical stack of at least a first upper, a second,third and a fourth bottom solar subcells, the fourth bottom subcellhaving a bottom contact connected to the terminal of second polarity.The fourth subcell of the first semiconductor body is connected in aseries electrical circuit with the fourth subcell of the secondsemiconductor body.

Some implementations include one or more of the following features. Forexample, in some cases, the upper first subcell of the firstsemiconductor body is composed of indium gallium phosphide (InGaP); thesecond solar subcell of the first semiconductor body is disposedadjacent to and lattice matched to said upper first subcell, the secondsolar subcell composed of aluminum gallium arsenide (AlGaAs) or indiumgallium arsenide phosphide (InGaAsP), and the third subcell is thebottom subcell of the first semiconductor body and is lattice matched tosaid second subcell and is composed of indium gallium arsenide (In)GaAs.

In some instances, the upper first subcell of the first semiconductorbody is composed of aluminium indium gallium phosphide (AlInGaP); thesecond solar subcell of the first semiconductor body is disposedadjacent to and lattice matched to said upper first subcell, and iscomposed of aluminum gallium arsenide (AlGaAs); and the third subcell isdisposed adjacent to and lattice matched to said second subcell and iscomposed of indium gallium arsenide (In)GaAs.

In some cases (e.g., for an assembly having two subassemblies), theshort circuit density (J_(sc)/cm²) of each of the first and secondsubcells is approximately 12 mA/cm². In other instances (e.g., for anassembly having three subassemblies), the short circuit density(J_(sc)/cm²) of each of the first, second and third middle subcells isapproximately 10 mA/cm². The short circuit density (J_(sc)/cm²) of thebottom subcell in the foregoing cases can be approximately greater than24 mA/cm². However, the short circuit densities (J_(sc)/cm²) may havedifferent values in some implementations.

In some embodiments, the fourth subcell is germanium.

In some embodiments, the fourth subcell is InGaAs, GaAsSb, InAsP,InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN,InGaBiN, InGaSbBiN.

In some embodiments, the second subcell has a band gap of approximately1.73 eV and the upper first subcell has a band gap of approximately 2.10eV.

In some embodiments, the upper first subcell is composed of indiumgallium aluminum phosphide; the second solar subcell includes an emitterlayer composed of indium gallium phosphide or aluminum gallium arsenide,and a base layer composed of aluminum gallium arsenide; the third solarsubcell is composed of indium gallium arsenide; and the fourth subcellis composed of germanium.

In some embodiments, there further comprises a distributed Braggreflector (DBR) layer adjacent to and between the third and the fourthsolar subcells and arranged so that light can enter and pass through thethird solar subcell and at least a portion of which can be reflectedback into the third solar subcell by the DBR layer.

In some embodiments, the distributed Bragg reflector layer is composedof a plurality of alternating layers of lattice matched materials withdiscontinuities in their respective indices of refraction.

In some embodiments, the difference in refractive indices betweenalternating layers is maximized in order to minimize the number ofperiods required to achieve a given reflectivity, and the thickness andrefractive index of each period determines the stop band and itslimiting wavelength.

In some embodiments, the DBR layer includes a first DBR layer composedof a plurality of n type or p type Al_(x)Ga_(1−x)As layers, and a secondDBR layer disposed over the first DBR layer and composed of a pluralityof n type or p type Al_(y)G_(1−y)As layers, where 0<x<1, 0<y<1, and y isgreater than x.

In another aspect, the present disclosure provides a five junction solarcell comprising a pair of semiconductor bodies, each body having adifferent sequence of semiconductor layers but each body including anupper first solar subcell composed of a semiconductor material having afirst band gap; a substantially identical second solar subcell adjacentto said first solar subcell and composed of a semiconductor materialhaving a second band gap smaller than the first band gap and beinglattice matched with the upper first solar subcell; a substantiallyidentical third solar subcell adjacent to said second solar subcell andcomposed of a semiconductor material having a third band gap smallerthan the second band gap and being lattice matched with the second solarsubcell; and a substantially identical fourth solar subcell adjacent toand lattice matched to said third solar subcell and composed of asemiconductor material having a fourth band gap smaller than the thirdband gap; wherein the average band gap of all four subcells (i.e., thesum of the four band gaps of each subcell divided by four) is greaterthan 1.44 eV.

In another aspect, the present disclosure provides a method ofmanufacturing a five junction solar cell assembly comprising providingtwo germanium substrates; growing on each germanium substrate a sequenceof layers of semiconductor material using a semiconductor dispositionprocess to form a solar cell comprising a plurality of subcellsincluding a third subcell disposed over the substrate and having a bandgap of approximately 1.41 eV, a second subcell disposed over the thirdsubcell and having a band gap in the range of approximately 1.65 to 1.8eV and an upper first subcell disposed over the second subcell andhaving a band gap in the range of 2.0 to 2.20 eV.

In some embodiments, there further comprises (i) a back surface field(BSF) layer disposed directly adjacent to the bottom surface of thethird subcell, and (ii) at least one distributed Bragg reflector (DBR)layer directly below the BSF layer so that light can enter and passthrough the first, second and third subcells and at least a portion ofwhich be reflected back into the third subcell by the DBR layer.

In some embodiments, the fourth (i.e., bottom) subcell of each of thesolar cell subassemblies is composed of germanium. The indirect band gapof the germanium at room temperature is about 0.66 eV, while the directband gap of germanium at room temperature is 0.8 eV. Those skilled inthe art with normally refer to the “band gap” of germanium as 0.66 eV,since it is lower than the direct band gap value of 0.8 eV. Thus, insome implementations, the fourth subcell has a direct band gap ofgreater than 0.75 eV. Reference to the fourth subcell having a directband gap of greater than 0.75 eV is expressly meant to include germaniumas a possible semiconductor material for the fourth subcell, althoughother semiconductor materials can be used as well. For example, thefourth subcell may be composed of InGaAs, GaAsSb, InAsP, InAlAs, orSiGeSn, or other III-V or II-VI compound semiconductor materials.

In some embodiments, additional layer(s) may be added or deleted in thecell structure without departing from the scope of the presentdisclosure.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingsummaries.

Additional aspects, advantages, and novel features of the presentdisclosure will become apparent to those skilled in the art from thisdisclosure, including the following detailed description as well as bypractice of the disclosure. While the disclosure is described below withreference to preferred embodiments, it should be understood that thedisclosure is not limited thereto. Those of ordinary skill in the arthaving access to the teachings herein will recognize additionalapplications, modifications and embodiments in other fields, which arewithin the scope of the disclosure as disclosed and claimed herein andwith respect to which the disclosure could be of utility.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better and more fully appreciated by reference tothe following detailed description when considered in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a graph representing the BOL value of the parameterE_(g)/q−V_(oc) at 28° C. plotted against the band gap of certain ternaryand quaternary materials defined along the x-axis;

FIG. 2A is a cross-sectional view of a first embodiment of a fivejunction solar cell assembly after several stages of fabricationincluding the growth of certain semiconductor layers on the growthsubstrate of up to the contact layer and etching contact steps on lowerlevels according to the present disclosure;

FIG. 2B is a cross-sectional view of a second embodiment of a fivejunction solar cell assembly after several stages of fabricationincluding the growth of certain semiconductor layers on the growthsubstrate of up to the contact layer and etching contact steps on lowerlevels according to the present disclosure;

FIG. 2C is a top plan view of two subassemblies being connected togetherto form a single solar cell assembly;

FIG. 3 is a schematic diagram of the five junction solar cell assemblyof FIG. 2B; and

FIG. 4 is a graph of the doping profile in the base and emitter layersof a subcell in the solar cell according to the present disclosure.

GLOSSARY OF TERMS

“III-V compound semiconductor” refers to a compound semiconductor formedusing at least one elements from group III of the periodic table and atleast one element from group V of the periodic table. III-V compoundsemiconductors include binary, tertiary and quaternary compounds. GroupIII includes boron (B), aluminum (Al), gallium (Ga), indium (In) andthallium (T). Group V includes nitrogen (N), phosphorus (P), arsenic(As), antimony (Sb) and bismuth

“Band gap” refers to an energy difference (e.g., in electron volts (eV))separating the top of the valence band and the bottom of the conductionband of a semiconductor material.

“Beginning of Life (BOL)” refers to the time at which a photovoltaicpower system is initially deployed in operation.

“Bottom subcell” refers to the subcell in a multijunction solar cellwhich is furthest from the primary light source for the solar cell.

“Compound semiconductor” refers to a semiconductor formed using two ormore chemical elements.

“Current density” refers to the short circuit current density J_(sc)through a solar subcell through a given planar area, or volume, ofsemiconductor material constituting the solar subcell.

“Deposited”, with respect to a layer of semiconductor material, refersto a layer of material which is epitaxially grown over anothersemiconductor layer.

“End of Life (EOL)” refers to a predetermined time or times after theBeginning of Life, during which the photovoltaic power system has beendeployed and has been operational. The EOL time or times may, forexample, be specified by the customer as part of the required technicalperformance specifications of the photovoltaic power system to allow thesolar cell designer to define the solar cell subcells and sublayercompositions of the solar cell to meet the technical performancerequirement at the specified time or times, in addition to other designobjectives. The terminology “EOL” is not meant to suggest that thephotovoltaic power system is not operational or does not produce powerafter the EOL time.

“Graded interlayer” (or “grading interlayer”)—see “metamorphic layer”.

“Inverted metamorphic multijunction solar cell” or “IMM solar cell”refers to a solar cell in which the subcells are deposited or grown on asubstrate in a “reverse” sequence such that the higher band gapsubcells, which are to be the “top” subcells facing the solar radiationin the final deployment configuration, are deposited or grown on agrowth substrate prior to depositing or growing the lower band gapsubcells, following which the growth substrate is removed leaving theepitaxial structure.

“Layer” refers to a relatively planar sheet or thickness ofsemiconductor or other material. The layer may be deposited or grown,e.g., by epitaxial or other techniques.

“Lattice mismatched” refers to two adjacently disposed materials orlayers (with thicknesses of greater than 100 nm) having in-plane latticeconstants of the materials in their fully relaxed state differing fromone another by less than 0.02% in lattice constant. (Applicant expresslyadopts this definition for the purpose of this disclosure, and notesthat this definition is considerably more stringent than that proposed,for example, in U.S. Pat. No. 8,962,993, which suggests less than 0.6%lattice constant difference).

“Metamorphic layer” or “graded interlayer” refers to a layer thatachieves a gradual transition in lattice constant generally throughoutits thickness in a semiconductor structure.

“Middle subcell” refers to a subcell in a multijunction solar cell whichis neither a Top Subcell (as defined herein) nor a Bottom Subcell (asdefined herein).

“Short circuit current (I_(sc))” refers to the amount of electricalcurrent through a solar cell or solar subcell when the voltage acrossthe solar cell is zero volts, as represented and measured, for example,in units of milliamps.

“Short circuit current density”—see “current density”.

“Solar cell” refers to an electro-optical semiconductor device operableto convert the energy of light directly into electricity by thephotovoltaic effect.

“Solar cell assembly” refers to two or more solar cell subassembliesinterconnected electrically with one another.

“Solar cell subassembly” refers to a stacked sequence of layersincluding one or more solar subcells.

“Solar subcell” refers to a stacked sequence of layers including a p-nphotoactive junction composed of semiconductor materials. A solarsubcell is designed to convert photons over different spectral orwavelength bands to electrical current.

“Substantially current matched” refers to the short circuit currentthrough adjacent solar subcells being substantially identical (i.e.within plus or minus 1%).

“Top subcell” or “upper subcell” refers to the subcell in amultijunction solar cell which is closest to the primary light sourcefor the solar cell.

“ZTJ” refers to the product designation of a commercially availableSolAero Technologies Corp. triple junction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Details of the present invention will now be described includingexemplary aspects and embodiments thereof. Referring to the drawings andthe following description, like reference numbers are used to identifylike or functionally similar elements, and are intended to illustratemajor features of exemplary embodiments in a highly simplifieddiagrammatic manner. Moreover, the drawings are not intended to depictevery feature of the actual embodiment nor the relative dimensions ofthe depicted elements, and are not drawn to scale.

A variety of different features of multijunction solar cells (as well asinverted metamorphic multijunction solar cells) are disclosed in therelated applications noted above. Some, many or all of such features maybe included in the structures and processes associated with thenon-inverted or “upright” solar cells of the present disclosure.However, more particularly, the present disclosure is directed to thefabrication of a multijunction lattice matched solar cell assemblyformed from the interconnection of two discrete and distinctsubassemblies. More specifically, however, in some embodiments, thepresent disclosure relates to multijunction solar cell subassemblieswith direct band gaps in the range of 2.0 to 2.15 eV (or higher) for thetop subcell, and (i) 1.65 to 1.8 eV, and (ii) 1.41 eV for the middlesubcells, and 0.6 to 0.8 eV direct or indirect band gaps, for the bottomsubcell, respectively, and the connection of two or more suchsubassemblies to form a solar cell assembly.

As described in greater detail, the present application notes thatinterconnecting two or more spatially split multijunction solar cellsubassemblies can be advantageous. The spatial split can be provided formultiple solar cell subassemblies monolithically formed on the samesubstrate. Alternatively, the solar cell subassemblies can be fabricatedas separate semiconductor chips that can be coupled togetherelectrically.

In general terms, a solar cell assembly in accordance with one aspect ofthe present disclosure, can include a terminal of first polarity and aterminal of second polarity. The solar cell assembly includes a firstsemiconductor subassembly including a tandem vertical stack of at leasta first upper, a second, third and fourth bottom solar subcells, thefirst upper subcell having a top contact connected to the terminal offirst polarity. A second semiconductor subassembly is disposed adjacentto the first semiconductor subassembly and includes a tandem verticalstack of at least a first upper, a second, third, and fourth bottomsolar subcells, the fourth bottom subcell having a back side contactconnected to the terminal of second polarity. The fourth subcell of thefirst semiconductor subassembly is connected in a series electricalcircuit with the third subcell of the second semiconductor subassembly.Thus, a five-junction solar assembly is assembled from two differentfour-junction solar cell subassemblies.

In some cases, the foregoing solar cell assembly can provide increasedphotoconversion efficiency in a multijunction solar cell for outer spaceor other applications over the operational life of the photovoltaicpower system.

Another aspect of the present disclosure is that to provide a fourjunction solar cell assembly composed of spatially separated solar cellsubassemblies, the average band gap of all four subcells (i.e., the sumof the four band gaps of each subcell divided by 4) in each solar cellsubassembly being greater than 1.44 eV.

Another descriptive aspect of the present disclosure is to characterizethe fourth subcell as being composed of an indirect or direct band gapmaterial such that the lowest direct band gap is greater than 0.75 eV,in some embodiments.

In some embodiments, the fourth subcell in each solar cell subassemblyis germanium, while in other embodiments the fourth subcell is InGaAs,GaAsSb, InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi,InGaAsNSbBi, InGaSbN, InGaBiN, InGaSbBiN or other III-V or II-VIcompound semiconductor material.

The indirect band gap of germanium at room temperature is about 0.66 eV,while the direct band gap of germanium at room temperature is 0.8 eV.Those skilled in the art will normally refer to the “band gap” ofgermanium as 0.66 eV, since it is lower than the direct band gap valueof 0.8 eV.

The recitation that “the fourth subcell has a direct band gap of greaterthan 0.75 eV” is therefore expressly meant to include germanium as apossible semiconductor for the fourth subcell, although othersemiconductor materials can be used as well.

More specifically, the present disclosure intends to provide arelatively simple and reproducible technique that does not employinverted processing associated with inverted metamorphic multijunctionsolar cells, and is suitable for use in a high volume productionenvironment in which various semiconductor layers are grown on a growthsubstrate in an MOCVD reactor, and subsequent processing steps aredefined and selected to minimize any physical damage to the quality ofthe deposited layers, thereby ensuring a relatively high yield ofoperable solar cells meeting specifications at the conclusion of thefabrication processes.

Prior to discussing the specific embodiments of the present disclosure,a brief discussion of some of the issues associated with the design ofmultijunction solar cells, and in particular metamorphic solar cells,and the context of the composition or deposition of various specificlayers in embodiments of the product as specified and defined byApplicant is in order.

There are a multitude of properties that should be considered inspecifying and selecting the composition of, inter alia, a specificsemiconductor layer, the back metal layer, the adhesive or bondingmaterial, or the composition of the supporting material for mounting asolar cell thereon. For example, some of the properties that should beconsidered when selecting a particular layer or material are electricalproperties (e.g. conductivity), optical properties (e.g., band gap,absorbance and reflectance), structural properties (e.g., thickness,strength, flexibility, Young's modulus, etc.), chemical properties(e.g., growth rates, the “sticking coefficient” or ability of one layerto adhere to another, stability of dopants and constituent materialswith respect to adjacent layers and subsequent processes, etc.), thermalproperties (e.g., thermal stability under temperature changes,coefficient of thermal expansion), and manufacturability (e.g.,availability of materials, process complexity, process variability andtolerances, reproducibility of results over high volume, reliability andquality control issues).

In view of the trade-offs among these properties, it is not alwaysevident that the selection of a material based on one of itscharacteristic properties is always or typically “the best” or “optimum”from a commercial standpoint or for Applicant's purposes. For example,theoretical studies may suggest the use of a quaternary material with acertain band gap for a particular subcell would be the optimum choicefor that subcell layer based on fundamental semiconductor physics. As anexample, the teachings of academic papers and related proposals for thedesign of very high efficiency (over 40%) solar cells may thereforesuggest that a solar cell designer specify the use of a quaternarymaterial (e.g., InGaAsP) for the active layer of a subcell. A few suchdevices may actually be fabricated by other researchers, efficiencymeasurements made, and the results published as an example of theability of such researchers to advance the progress of science byincreasing the demonstrated efficiency of a compound semiconductormultijunction solar cell. Although such experiments and publications areof “academic” interest, from the practical perspective of the Applicantsin designing a compound semiconductor multijunction solar cell to beproduced in high volume at reasonable cost and subject to manufacturingtolerances and variability inherent in the production processes andsuited for specific applications such as the space environment where theefficiency over the entire operational life is an important goal, suchan “optimum” design from an academic perspective is not necessarily themost desirable design in practice, and the teachings of such studiesmore likely than not point in the wrong direction and lead away from theproper design direction. Stated another way, such references mayactually “teach away” from Applicant's research efforts and the ultimatesolar cell design proposed by the Applicants.

In view of the foregoing, it is further evident that the identificationof one particular constituent element (e.g. indium, or aluminum) in aparticular subcell, or the thickness, band gap, doping, or othercharacteristic of the incorporation of that material in a particularsubcell, is not a “result effective variable” that one skilled in theart can simply specify and incrementally adjust to a particular leveland thereby increase the efficiency of a solar cell at the beginning oflife or the end of life. The efficiency of a solar cell is not a simplelinear algebraic equation as a function of the amount of gallium oraluminum or other element in a particular layer. The growth of each ofthe epitaxial layers of a solar cell in an MOCVD reactor is anon-equilibrium thermodynamic process with dynamically changing spatialand temporal boundary conditions that is not readily or predictablymodeled. The formulation and solution of the relevant simultaneouspartial differential equations covering such processes are not withinthe ambit of those of ordinary skill in the art in the field of solarcell design.

Even when it is known that particular variables have an impact onelectrical, optical, chemical, thermal or other characteristics, thenature of the impact often cannot be predicted with much accuracy,particularly when the variables interact in complex ways, leading tounexpected results and unintended consequences. Thus, significant trialand error, which may include the fabrication and evaluative testing ofmany prototype devices, often over a period of time of months if notyears, is required to determine whether a proposed structure with layersof particular compositions, actually will operate as intended, in agiven environment over the operational life, let alone whether it can befabricated in a reproducible high volume manner within the manufacturingtolerances and variability inherent in the production process, andnecessary for the design of a commercially viable device.

Furthermore, as in the case here, where multiple variables interact inunpredictable ways, the proper choice of the combination of variablescan produce new and “unexpected results”, and constitute an “inventivestep” in designing and specifying a solar cell to operate in apredetermined environment (such as space), not only at the beginning oflife, but over the entire defined operational lifetime.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

One aspect of the present disclosure relates to the use of aluminum inthe active layers of the upper subcells in a multijunction solar cell.The effects of increasing amounts of aluminum as a constituent elementin an active layer of a subcell affects the photovoltaic deviceperformance. One measure of the “quality” or “goodness” of a solar cellsubcell or junction is the difference between the band gap of thesemiconductor material in that subcell or junction and the V_(oc), oropen circuit voltage, of that same junction. The smaller the difference,the higher the V_(oc) of the solar cell junction relative to the bandgap, and the better the performance of the device. V_(oc) is verysensitive to semiconductor material quality, so the smaller theE_(g)/q−V_(oc) of a device, the higher the quality of the material inthat device. There is a theoretical limit to this difference, known asthe Shockley-Queisser limit. That is the best that a solar cell junctioncan be under a given concentration of light at a given temperature.

The experimental data obtained for single junction (Al)GaInP solar cellsindicates that increasing the Al content of the junction leads to alarger V_(oc)−E_(g)/q difference, indicating that the material qualityof the junction decreases with increasing Al content. FIG. 1 shows thiseffect. The three compositions cited in the Figure are all latticematched to GaAs, but have differing Al composition. As seen by thedifferent compositions represented, with increasing amount of aluminumrepresented by the x-axis, adding more Al to the semiconductorcomposition increases the band gap of the junction, but in so doing alsoincreases V_(oc)−E_(g)/q. Hence, we draw the conclusion that adding Alto a semiconductor material degrades that material such that a solarcell device made out of that material does not perform relatively aswell as a junction with less Al.

The lattice constants and electrical properties of the layers in thesemiconductor structure are preferably controlled by specification ofappropriate reactor growth temperatures and times, and by use ofappropriate chemical composition and dopants. The use of a depositionmethod, such as Molecular Beam Epitaxy (MBE), Organo Metallic VaporPhase Epitaxy (OMVPE), Metal Organic Chemical Vapor Deposition (MOCVD),or other vapor deposition methods for the growth may enable the layersin the monolithic semiconductor structure forming the cell to be grownwith the required thickness, elemental composition, dopant concentrationand grading and conductivity type.

The present disclosure is directed to, in one embodiment, a growthprocess using a metal organic chemical vapor deposition (MOCVD) processin a standard, commercially available reactor suitable for high volumeproduction. More particularly, the present disclosure is directed to thematerials and fabrication steps that are particularly suitable forproducing commercially viable multijunction solar cells usingcommercially available equipment and established high-volume fabricationprocesses, as contrasted with merely academic expositions of laboratoryor experimental results.

Turning to the fabrication of the multijunction solar cell assembly ofthe present disclosure, and in particular a five-junction solar cellassembly, FIG. 2A is a cross-sectional view of a first embodiment of asolar cell assembly comprising two four junction solar cellsubassemblies 100 and 200 after several stages of fabrication includingthe growth of certain semiconductor layers on the growth substrate, andformation of grids and contacts on the contact layers of thesemiconductor bodies, and interconnection of the two subassemblies 100and 200.

As illustrated in FIG. 2A, similar to that presented in relatedapplication Ser. No. 15/213,594, a first solar cell subassembly 100includes multiple solar subcells in a tandem stack. In the illustratedexample, the subassembly 100 includes an upper first subcell 107(Subcell A₁), a second middle solar subcell 106 (Subcell B₁) disposedadjacent to and lattice matched to the upper first subcell 107, a thirdmiddle subcell 108 (Subcell C₁), and a bottom subcell 101 (Subcell D₁)lattice matched to the third subcell 105. In the illustrated example,the subcells 101, 105, 106, 107 are configured so that the short circuitcurrent densities of the upper first subcell 107, the second subcell106, and the third subcell 105 have a substantially equal predeterminedfirst value (J1=J2=J3), and the short circuit current density (J4) ofthe bottom subcell 101 is at least twice that of the predetermined firstvalue.

In the example of FIG. 2A, the upper first subcell 107 is composed of(aluminum) indium gallium phosphide ((Al)InGaP), the second solarsubcell 106 is composed of (aluminum) gallium arsenide ((Al)GaAs) orindium gallium arsenide phosphide (InGaAsP), and the bottom subcell 101is composed of germanium (Ge) or gallium arsenide (GaAs). Each of thesubcells includes a respective junction formed, respectively, by p typeand n+ type regions of the semiconductor material for the particularsubcell. Thus, for example, the upper subcell 107 includes adjacent pand n+ regions 115, 116 of (Al)InGaP. Likewise, the second subcell 106includes adjacent p and n+ regions 110, 111 of AlGaAs or InGaAsP.Similarly, the bottom subcell 101 includes adjacent p and n+ regions102, 103 of Ge or GaAs.

The first solar cell subassembly 100 of FIG. 2A can include additionalsemiconductor layers as well, such as highly doped lateral conductionlayers 104A, 104B and a blocking p-n diode or insulating layer 105disposed between the first and second subcells 107, 106. In this case,the blocking p-n diode or insulating layer 105 is adjacent to, andsandwiched between, the highly doped lateral conduction layers 104A,104B. Thus, the first highly doped lateral conduction layer 104A isdisposed adjacent to and beneath the blocking p-n diode or insulatinglayer 105. Likewise, the blocking p-n diode or insulating layer 105 isdisposed adjacent to and beneath the second highly doped lateralconduction layer 104B.

In some implementations, such as a triple junction solar cell, the bandgap of the first upper subcell 107 is in the range of 1.85 to 1.95 eV,the band gap of the second subcell 106 is in the range of 1.4 to 1.5 eV,and the band gap of the bottom subcell 201 is in the range of 0.6 to 0.8eV.

In some implementations, in a four junction device, the band gap of thefirst upper subcell 107 is 2.0 to 2.2 eV, the band gap of the secondsubcell 106 is in the range of 1.65 to 1.8 eV, and the band gap of thethird solar cell is 1.41 eV, and the band gap of the bottom subcell 201is in the range of 0.6 to 0.8 eV. In such an implementation, the averageband gap of the top three subcells is at least 1.44 eV.

The solar cell subassembly 100 also includes electrically conductivecontacts (see, e.g., metallization 117) on the bottom of the subcell101.

As described in greater detail below, different layers in the solar cellsubassembly 100 can be connected electrically to one another. Further insome cases, two or more spatially separated multijunction solar cellsubassemblies can be connected together electrically, for example,through electrically conductive interconnects. In order to provideaccess to the various layers so as to facilitate such connections,various ones of the layers in the solar cell subassembly 100 can beexposed partially. Thus, as shown in the example of FIG. 1A, variouslayers are partially exposed, for example, using standardphotolithographic etching techniques to etch from the top surface of thesubassembly 100 to the particular contact layer(s) 120, 121, 122 ofinterest (i.e., the bottom contact layer 122 for the second subcell 106;the bottom contact layer 121 for blocking p-n diode or insulating layer105; and the bottom contact layer 120 for the n+ layer 103 of the bottomsubcell 101).

On the right hand side of FIG. 2A there is illustrated the second solarcell subassembly 200, which is similar to the solar cell subassembly100. The second solar cell subassembly 200 can have substantially thesame sequence of semiconductor layers with the same compositions andbandgaps as the corresponding layers in the first solar cell subassembly100. Thus, the solar cell subassembly 200 also includes multiplesubcells in a tandem stack. In the illustrated example of FIG. 2A, thesecond solar cell subassembly 200 includes an upper first subcell 207(Subcell A2), a second solar subcell 206 (Subcell B2) disposed adjacentto and lattice matched to the upper first subcell 207, and a bottomsubcell 201 (Subcell D) lattice matched to the second subcell 206. Aswith the first solar cell subassembly 100, the subcells 201, 206, 207 ofthe second solar cell subassmbly can be configured so that the shortcircuit current densities of the upper first subcell 207 and the secondsubcell 206 have a substantially equal predetermined first value(J1=J2), and the short circuit current density (J3) of the bottomsubcell 201 is at least twice that of the predetermined first value.

Referring to example of FIG. 2A, the upper first subcell 207 is composedof (aluminum) indium gallium phosphide ((Al)InGaP), the second solarsubcell 206 is composed of (aluminum) gallium arsenide ((Al)GaAs) orindium gallium arsenide phosphide (InGaAsP), and the bottom subcell 201is composed of germanium (Ge) or other suitable semiconductor material.Each of the subcells includes a respective junction formed,respectively, by p type and n+ type regions of the semiconductormaterial for the particular subcell. Thus, for example, the uppersubcell 207 includes adjacent p and n+ regions 215, 216 of (A1)InGaP.Likewise, the second subcell 206 includes adjacent p and n+ regions 210,211 of (A1)GaAs or InGaAsP. Similarly, the bottom subcell 201 includesadjacent p and n+ regions 202, 203 of Ge or other suitable semiconductormaterial.

The second solar cell subassembly 200 also can include a blocking p-ndiode or insulating layer 205 sandwiched between first and second highlydoped lateral conduction layers 204A, 204B. Electrically contacts (e.g.,216 and 217) can be provided, respectively, making electrical contactwith the top and bottom subcells 207, 201.

In order to provide access to the various layers in the second solarcell subassembly 200, various ones of the layers can be exposedpartially. Thus, as shown in the example of FIG. 2A, various surfacesare partially exposed, for example, using standard photolithographicetching techniques to etch from the top surface of the semiconductorbody 200 to the particular contact layer 221, 222 of interest (i.e., thebottom contact layer 222 for the second subcell 206; and the bottomcontact layer 221 for blocking p-n diode or insulating layer 205).

FIG. 2A also illustrates the metal contact pads on the ledges 120, 121,122, 221 and 222 depicted in FIG. 2A.

A metal contact pad 132 is deposited on the surface of the ledge of 122which exposes a portion of the top surface of the lateral conductionlayer 104 b. This pad 132 allows electrical contact to be made to thebottom of the stack of subcells A₁ through C₁ on subassembly 100.

Similarly, a metal contact pad 232 is deposited on the surface of theledge of 222 which exposes a portion of the top surface of the lateralconduction layer 204 b. This pad 232 allows electrical contact to bemade to the bottom of the stack of subcells A₂ through C₂ on subassembly200.

A metal contact pad 131 is deposited on the surface of the ledge of 121which exposes a portion of the top surface of the lateral conductionlayer 104 a. This pad 131 allows electrical contact to be made to thetop of the subcell D₁.

A metal contact pad 231 is deposited on the surface of the ledge of 221which exposes a portion of the top surface of the lateral conductionlayer 204 a. This pad 231 allows electrical contact to be made to thetop of the subcell D₂.

A metal contact pad 130 is further provided on the surface of ledge 120which allows electrical contact to be made to the p-terminal of subcellD₁.

The foregoing multijunction solar cell subassemblies 100 or 200 can befabricated, for example, in wafer-level processes and then diced intoindividual semiconductor chips. The various semiconductor layers can begrown, one atop another, using known growth techniques (e.g., MOCVD) asdiscussed above.

Each solar cell subassembly 100, 200 also can include grid lines,interconnecting bus lines, and contact pads. FIG. 2 of Ser. No.15/213,594 illustrates an example of a top view of the solar cellsubassembly 100, which includes grid lines 140, interconnecting buslines 142, and electrically conductive contacts 143, 144, 145, 146. Thesolar cell subassembly 200 can include similar grid lines,interconnecting bus lines, and contact pads. The geometry and number ofthe grid lines, bus lines and/or contacts may vary in otherimplementations.

In some embodiments, the bottom subcell D₁ and D₂ is germanium, while inother embodiments the fourth subcell is InGaAs, GaAsSb, InAsP, InAlAs,or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN,InGaSbBiN or other III-V or II-VI compound semiconductor material.

The bottom subcell D₁ and D₂ further includes, for example, a highlydoped n-type Ge emitter layer 102, 103, and an n-type indium galliumarsenide (“InGaAs”) nucleation layer. The nucleation layer is depositedover the substrate, and the emitter layer 103, 203 is formed in thesubstrate by diffusion of deposits into the Ge substrate, therebyforming the n-type Ge layer 103, 203.

In the solar cell subassemblies 100 and 200 of FIG. 2A, a highly dopedlateral conduction layer 104A, 204A is deposited over layer 103, 203,and a blocking p-n diode or insulating layer 105, 205 is deposited overthe layer 104A, 204A, respectively. A highly doped lateral conductionlayer 602 d is then deposited over later 602 c.

Turning to the fabrication of the multijunction solar cell assembly ofthe present disclosure, and in particular a five-junction solar cellassembly, FIG. 2B is a cross-sectional view of a second embodiment of asolar cell assembly comprising two four junction solar cellsubassemblies 300 and 200 after several stages of fabrication includingthe growth of certain semiconductor layers on the growth substrate, andformation of grids and contacts on the contact layers of thesemiconductor bodies, and interconnection of the two subassemblies 300and 200.

The second embodiment depicted in FIG. 2B includes a left subassembly300 which has a sequence of layers which is substantially identical tosubassembly 100 of FIG. 2A, except that layers 105 and 104B are omitted,while the right subassembly 200 depicted in FIG. 2B is identical tosubassembly 200 of FIG. 2A. Therefore in the interest of brevity of thisdisclosure, the description of the identical layers in left subassembly300 and right subassembly 200 will not be repeated here.

As with the first solar cell subassembly 100 or 300, the subcells A₂,B₂, C₂ of the second solar cell subassembly 200 can be configured sothat the short circuit current densities of the three subcells A₂, B₂,C₂ have a substantially equal predetermined first value (J1=J2=J3), andthe short circuit current density (J4) of the bottom subcell E is atleast twice that of the predetermined first value.

The foregoing multijunction solar cell subassemblies 100, 200, or 300can be fabricated, for example, in one or two distinct wafer-levelprocesses and then diced into individual semiconductor chips. Thevarious semiconductor layers can be grown, one atop another, using knowngrowth techniques (e.g., MOCVD) as discussed above.

Each solar cell subassembly 100, 200, 300 also can include grid lines,interconnecting bus lines, and contact pads. The geometry and number ofthe grid lines, bus lines and/or contacts may vary in otherimplementations.

As previously mentioned, two (or more) solar cell subasemblies (e.g.,300 and 200) can be connected together electrically. For example, asshown in FIGS. 2A and 2B, conductive (e.g., metal) interconnections 801,802, 803, and 804 can be made between different layers of the solar cellsubassemblies 100 and 200, and 300 and 200. Some of the interconnectionsare made between different layers of a single one of the solar cellsubassemblies, whereas others of the interconnections are made betweenthe two different solar cell subassemblies. Thus, for example, theinterconnection 801 electrically connects together the metal contacts133 and 233 of the first and second solar cell subassemblies 100, 300and 200 respectively. In particular, interconnection 803 connectstogether a contact 132 on the lateral conduction layer 104 b of thefirst solar cell subassembly 100 to a contact 232 on the lateralconduction layer 204 b of the second solar cell subassembly 200.Similarly, the interconnection 804 connects together a contact 130 onthe p-base layer 102 of the first solar cell subassembly 100 to acontact 231 on the lateral conduction layer 204 a of the second solarcell subassembly 200. Likewise, the interconnection 802 connectstogether a contact 132 on the lateral conduction layer 104 b of thefirst solar cell subassembly 100 to a contact 131 on the lateralconduction layer 104 a of the first solar cell subassembly 100.

In some instances, multiple electrically conductive (e.g., metal)contacts can be provided for each of the respective contacts of thesolar cell subassemblies 100, 200. This allows each of theinterconnections 801-804 to be implemented by multiple interconnectionsbetween the solar cell subassembly layers rather than just a singleinterconnection.

As noted above, the solar cell assembly includes a first electricalcontact of a first polarity and a second electrical contact of a secondpolarity. In some embodiments, the first electrical contact 807 isconnected to the metal contact 107 on the first solar cell subassembly100, 300 by an interconnection 805, and the second electrical contact808 is connected to the back metal contact 217 of subcell D₂ of thesecond solar cell subassembly 200.

As illustrated in FIG. 2B, two or more solar cell subassemblies can beconnected electrically as described above to obtain a multijunction(e.g. a four-, five- or six-junction) solar cell assembly. In FIG. 2B,the top side (n-polarity) conductivity contact 807 and bottom side(p-polarity) conductive contact 808 for the solar cell assembly areschematically depicted respectively, at the left and right-hand sides ofthe assembly.

In the example of FIG. 2B, one solar cell subassembly 300 includes anupper subcell A₁, two middle subcells B₁, C₁ and a bottom subcell D₁.The other solar cell subassembly 200 includes an upper subcell A₂, twomiddle subcells B₂, C₂ and a bottom subcell D₂. In some implementations,each solar cell subassembly 300, 200 has band gaps in the range of 2.0to 2.20 eV (or higher) for the top subcell, and (i) 1.65 to 1.8, and(ii) 1.41 eV for the middle subcells, and 0.6 to 0.8 eV, for the bottomsubcell, respectively, Further, in some embodiments, the average bandgap of all four subcells (i.e., the sum of the four band gaps of eachsubcell divided by four) in a given solar cell subassembly 500 or 700 isgreater than 1.44 eV. Other band gap ranges may be appropriate for someimplementations.

In some instances, the fourth (i.e., bottom) subcell is composed ofgermanium. The indirect band gap of the germanium at room temperature isabout 0.66 eV, while the direct band gap of germanium at roomtemperature is 0.8 eV. Those skilled in the art with normally refer tothe “band gap” of germanium as 0.66 eV, since it is lower than thedirect band gap value of 0.8 eV. Thus, in some implementations, thefourth subcell has a direct band gap of greater than 0.75 eV. Referenceto the fourth subcell having a direct band gap of greater than 0.75 eVis expressly meant to include germanium as a possible semiconductormaterial for the fourth subcell, although other semiconductor materialscan be used as well. For example, the fourth subcell may be composed ofInGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn, or other III-V or II-VIcompound semiconductor materials.

FIG. 2C is a top plan view of the two subassemblies 300 and 200 beingconnected together to form a single solar cell assembly. As illustratedin FIG. 2C, two (or in other embodiments, more) solar cell subassembliescan be connected electrically as described above to obtain amulti-junction (e.g., a four-, five- or six-junction) solar cellassembly. In FIG. 2C, the top side (n) conductive contacts 807 a, 807 band bottom side (p) conductive contacts 808 a, 808 b for the solar cellassembly are visible, respectively, at the left and right-hand sides ofthe assembly, corresponding to contacts 807 and 808 depicted in FIG. 2B.

Similarly, interconnects 801, 803 and 804 shown in FIG. 2B are shown inan embodiment of two parallel interconnects, i.e., 801 a and 801 b for801, 803 a and 803 b for 803, and 804 a and 805 b for 804.

FIG. 3 is a schematic diagram of the five junction solar cell assemblyof FIG. 2B.

In some implementations of a five-junction solar cell assembly, such asin the example of FIG. 3, the short circuit density (J_(sc)) of theupper first subcells (A₁, and A₂) and the middle subcells (B₁, B₂, C₁,C₂) is about 12 mA/cm², and the short circuit current density (J_(sc))of the bottom subcells (D₁ and D₂) is about 34 mA/cm². Otherimplementations may have different values.

Some implementations provide that at least the base of at least one ofthe first, second or third solar subcells has a graded doping, i.e., thelevel of doping varies from one surface to the other throughout thethickness of the base layer. In some embodiments, the gradation indoping is exponential. In some embodiments, the gradation in doping isincremental and monotonic.

In some embodiments, the emitter of at least one of the first, second orthird solar subcells also has a graded doping, i.e., the level of dopingvaries from one surface to the other throughout the thickness of theemitter layer. In some embodiments, the gradation in doping is linear ormonotonically decreasing.

As a specific example, the doping profile of the emitter and base layersmay be illustrated in FIG. 4, which depicts the amount of doping in theemitter region and the base region of a subcell. N-type dopants includesilicon, selenium, sulfur, germanium or tin. P-type dopants includesilicon, zinc, chromium, or germanium.

In the example of FIG. 4, in some embodiments, one or more of thesubcells have a base region having a gradation in doping that increasesfrom a value in the range of 1×10¹⁵ to 1×10¹⁸ free carriers per cubiccentimeter adjacent the p-n junction to a value in the range of 1×10¹⁶to 4×10¹⁸ free carriers per cubic centimeter adjacent to the adjoininglayer at the rear of the base, and an emitter region having a gradationin doping that decreases from a value in the range of approximately5×10¹⁸ to 1×10¹⁷ free carriers per cubic centimeter in the regionimmediately adjacent the adjoining layer to a value in the range of5×10¹⁵ to 1×10¹⁸ free carriers per cubic centimeter in the regionadjacent to the p-n junction.

The heavy line shown in FIG. 4 illustrates one embodiment of the basedoping having an exponential gradation, and the emitter doping beinglinear.

Thus, the doping level throughout the thickness of the base layer may beexponentially graded from the range of 1×10¹⁶ free carriers per cubiccentimeter to 1×10¹⁸ free carriers per cubic centimeter, as representedby the curve 603 depicted in the Figure.

Similarly, the doping level throughout the thickness of the emitterlayer may decline linearly from 5×10¹⁸ free carriers per cubiccentimeter to 5×10¹⁷ free carriers per cubic centimeter as representedby the curve 602 depicted in the Figure.

The absolute value of the collection field generated by an exponentialdoping gradient exp [−x/λ] is given by the constant electric field ofmagnitude E=kT/q(1/λ))(exp[−x_(b)/λ]), where k is the Boltzman constant,T is the absolute temperature in degrees Kelvin, q is the absolute valueof electronic change, and λ is a parameter characteristic of the dopingdecay.

The efficacy of an embodiment of the doping arrangement presentdisclosure has been demonstrated in a test solar cell which incorporatedan exponential doping profile in the three micron thick base layer asubcell, according to one embodiment.

The exponential doping profile taught by one embodiment of the presentdisclosure produces a constant field in the doped region. In theparticular multijunction solar cell materials and structure of thepresent disclosure, the bottom subcell has the smallest short circuitcurrent among all the subcells. Since in a multijunction solar cell, theindividual subcells are stacked and form a series circuit, the totalcurrent flow in the entire solar cell is therefore limited by thesmallest current produced in any of the subcells. Thus, by increasingthe short circuit current in the bottom cell, the current more closelyapproximates that of the higher subcells, and the overall efficiency ofthe solar cell is increased as well. In a multijunction solar cell withapproximately efficiency, the implementation of the present dopingarrangement would thereby increase efficiency. In addition to anincrease in efficiency, the collection field created by the exponentialdoping profile will enhance the radiation hardness of the solar cell,which is important for spacecraft applications.

Although the exponentially doped profile is the doping design which hasbeen implemented and verified, other doping profiles may give rise to alinear varying collection field which may offer yet other advantages.For example, another doping profile may produce a linear field in thedoped region which would be advantageous for both minority carriercollection and for radiation hardness at the end-of-life (EOL) of thesolar cell. Such other doping profiles in one or more base layers arewithin the scope of the present disclosure.

The doping profile depicted herein are merely illustrative, and othermore complex profiles may be utilized as would be apparent to thoseskilled in the art without departing from the scope of the presentinvention.

Some implementations provide that a quantum well structure is includedin subcell C. Quantum well structures in multijunction solar cells areknown from U.S. patent application Ser. No. 11/788,315, filed Apr. 18,2007 hereby incorporated by reference.

In some embodiments, the plurality of quantum layers are “strainedbalanced” by incorporating alternating lower band gap (or larger latticeconstant) compressively strained InGaAs and higher band gap (or smallerlattice constant) tensionally strained GaAsP layers so that thelarger/smaller atomic lattices/layers of epitaxy balance the strain tokeep the quantum well layers lattice matched to the substrate.

In some embodiments, the number of quantum well layers are between 100and 300, which each layer being between 100 and 300 angstroms inthickness.

In some embodiments, the quantum well layers form an intermediate bandgap layer between the emitter layer and the base layer of the secondmiddle subcell.

In some embodiments, the total thickness of the quantum well layers isbetween two and four microns.

The present disclosure like that of the related parallel applications,U.S. patent application Ser. Nos. 14/828,206; 15/203, 975; and15/213,594, provides a multijunction solar cell that follows a designrule that one should incorporate as many high band gap subcells aspossible to achieve the goal to increase the efficiency at hightemperature EOL. For example, high band gap subcells may retain agreater percentage of cell voltage as temperature increases, therebyoffering lower power loss as temperature increases. As a result, bothhigh temperature beginning-of-life (HT-BOL) and HT-EOL performance ofthe exemplary multijunction solar cell, according to the presentdisclosure, may be expected to be greater than traditional cells.

The open circuit voltage (V_(oc)) of a compound semiconductor subcellloses approximately 2 mV per degree C. as the temperature rises, so thedesign rule taught by the present disclosure takes advantage of the factthat a higher band gap (and therefore higher voltage) subcell loses alower percentage of its V_(oc) with temperature. For example, a subcellthat produces a 1.50V at 28° C. produces 1.50−42*(0.0023)=1.403V at 70°C. which is a 6.4% voltage loss, A cell that produces 0.25V at 28° C.produces 0.25−42*(0.0018)=0.174V at 70° which is a 30.2% voltage loss.

In view of different satellite and space vehicle requirements in termsof temperature, radiation exposure, and operational life, a range ofsubcell designs using the design principles of the present disclosuremay be provided satisfying typical customer and mission requirements,and several embodiments are set forth hereunder, along with thecomputation of their efficiency at the end-of-life. The radiationexposure is experimentally measured using 1 MeV electron fluence persquare centimeter (abbreviated in the text that follows as e/cm²), sothat a comparison can be made between the current commercial devices andembodiments of solar cells discussed in the present disclosure.

As an example, a low earth orbit (LEO) satellite will typicallyexperience radiation equivalent to 5×10¹⁴ e/cm² over a five yearlifetime. A geosynchronous earth orbit (GEO) satellite will typicallyexperience radiation in the range of 5×10¹⁴ e/cm² to 1×10 e/cm² over afifteen year lifetime.

For example, the cell efficiency (%) measured at room temperature (RT)28° C. and high temperature (HT) 70° C., at beginning of life (BOL) andend of life (EOL), for a standard three junction commercial solar cell(e.g. a SolAero Technologies Corp. Model ZTJ), such as depicted in FIG.2 of U.S. patent application Ser. No. 14/828,206, is as follows:

Condition Efficiency BOL 28° C. 29.1% BOL 70° C. 26.4% EOL 70° C. 23.4%After 5E14 e/cm² radiation EOL 70° C. 22.0% After 1E15 e/cm² radiation

For the 5J solar cell assembly described in the present disclosure, thecorresponding data is as follows:

Condition Efficiency BOL 28° C. 30.6% BOL 70° C. 27.8% EOL 70° C. 26.6%After 5E14 e/cm² radiation EOL 70° C. 26.1% After 1E15 e/cm² radiation

The new solar cell has a slightly higher cell efficiency than thestandard commercial solar cell (ZTJ) at BOL at 70° C. However, the solarcell described in the present disclosure exhibits substantially improvedcell efficiency (%) over the standard commercial solar cell (ZTJ) at 1MeV electron equivalent fluence of 5×10¹⁴ e/cm², and dramaticallyimproved cell efficiency (%) over the standard commercial solar cell(ZTJ) at 1 MeV electron equivalent fluence of 1×10¹⁵ e/cm².

The wide range of electron and proton energies present in the spaceenvironment necessitates a method of describing the effects of varioustypes of radiation in terms of a radiation environment which can beproduced under laboratory conditions. The methods for estimating solarcell degradation in space are based on the techniques described by Brownet al. [Brown, W. L., J. D. Gabbe, and W. Rosenzweig, Results of theTelstar Radiation Experiments, Bell System Technical J., 42, 1505, 1963]and Tada [Tada, H. Y., J.R. Carter, Jr., B. E. Anspaugh, and R. G.Downing, Solar Cell Radiation Handbook, Third Edition, JPL Publication82-69, 1982]. In summary, the omnidirectional space radiation isconverted to a damage equivalent unidirectional fluence at a normalisedenergy and in terms of a specific radiation particle. This equivalentfluence will produce the same damage as that produced by omnidirectionalspace radiation considered when the relative damage coefficient (RDC) isproperly defined to allow the conversion. The relative damagecoefficients (RDCs) of a particular solar cell structure are measured apriori under many energy and fluence levels. When the equivalent fluenceis determined for a given space environment, the parameter degradationcan be evaluated in the laboratory by irradiating the solar cell withthe calculated fluence level of unidirectional normally incident flux.The equivalent fluence is normally expressed in terms of 1 MeV electronsor 10 MeV protons.

The software package Spenvis (www.spenvis.oma.be) is used to calculatethe specific electron and proton fluence that a solar cell is exposed toduring a specific satellite mission as defined by the duration,altitude, azimuth, etc. Spenvis employs the EQFLUX program, developed bythe Jet Propulsion Laboratory (JPL) to calculate 1 MeV and 10 MeV damageequivalent electron and proton fluences, respectively, for exposure tothe fluences predicted by the trapped radiation and solar proton modelsfor a specified mission environment duration. The conversion to damageequivalent fluences is based on the relative damage coefficientsdetermined for multijunction cells [Marvin, D. C., Assessment ofMultijunction Solar Cell Performance in Radiation Environments,Aerospace Report No. TOR-2000 (1210)-1, 2000]. A widely accepted totalmission equivalent fluence for a geosynchronous satellite mission of 15year duration is 1 MeV 1×10¹⁵ electrons/cm².

The exemplary solar cell described herein may require the use ofaluminum in the semiconductor composition of each of the top twosubcells. Aluminum incorporation is widely known in the III-V compoundsemiconductor industry to degrade BOL subcell performance due to deeplevel donor defects, higher doping compensation, shorter minoritycarrier lifetimes, and lower cell voltage and an increased BOLE_(g)/q−V_(oc) metric. In short, increased BOL E_(g)/q−V_(oc) may be themost problematic shortcoming of aluminum containing subcells; the otherlimitations can be mitigated by modifying the doping schedule orthinning base thicknesses.

It will be understood that each of the elements described above, or twoor more together, also may find a useful application in other types ofstructures or constructions differing from the types of structures orconstructions described above.

Although described embodiments of the present disclosure utilizes avertical tandem stack of four subcells, various aspects and features ofthe present disclosure can apply to tandem stacks with fewer or greaternumber of subcells, i.e. two junction cells, three junction cells, fivejunction cells, etc.

In addition, although the disclosed embodiments are configured with topand bottom electrical contacts, the subcells may alternatively becontacted by means of metal contacts to laterally conductivesemiconductor layers between the subcells. Such arrangements may be usedto form 3-terminal, 4-terminal, and in general, n-terminal devices. Thesubcells can be interconnected in circuits using these additionalterminals such that most of the available photogenerated current densityin each subcell can be used effectively, leading to high efficiency forthe multijunction cell, notwithstanding that the photogenerated currentdensities are typically different in the various subcells.

As noted above, the solar cell described in the present disclosure mayutilize an arrangement of one or more, or all, homojunction cells orsubcells, i.e., a cell or subcell in which the p-n junction is formedbetween a p-type semiconductor and an n-type semiconductor both of whichhave the same chemical composition and the same band gap, differing onlyin the dopant species and types, and one or more heterojunction cells orsubcells. Subcell A, with p-type and n+ type InGaAlP is one example of ahomojunction subcell.

In some cells, a thin so-called “intrinsic layer” may be placed betweenthe emitter layer and base layer, with the same or different compositionfrom either the emitter or the base layer. The intrinsic layer mayfunction to suppress minority-carrier recombination in the space-chargeregion. Similarly, either the base layer or the emitter layer may alsobe intrinsic or not-intentionally-doped (“NID”) over part or all of itsthickness.

The composition of the window or BSF layers may utilize othersemiconductor compounds, subject to lattice constant and band gaprequirements, and may include AlInP, AlAs, AlP, AlGaInP, AlGaAsP,AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs,GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AIN, GaN, InN,GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials,and still fall within the spirit of the present invention.

While the solar cell described in the present disclosure has beenillustrated and described as embodied in a conventional multijunctionsolar cell, it is not intended to be limited to the details shown, sinceit is also applicable to inverted metamorphic solar cells, and variousmodifications and structural changes may be made without departing inany way from the spirit of the present invention.

Thus, while the description of the semiconductor device described in thepresent disclosure has focused primarily on solar cells or photovoltaicdevices, persons skilled in the art know that other optoelectronicdevices, such as thermophotovoltaic (TPV) cells, photodetectors andlight-emitting diodes (LEDS), are very similar in structure, physics,and materials to photovoltaic devices with some minor variations indoping and the minority carrier lifetime. For example, photodetectorscan be the same materials and structures as the photovoltaic devicesdescribed above, but perhaps more lightly-doped for sensitivity ratherthan power production. On the other hand LEDs can also be made withsimilar structures and materials, but perhaps more heavily-doped toshorten recombination time, thus radiative lifetime to produce lightinstead of power. Therefore, this invention also applies tophotodetectors and LEDs with structures, compositions of matter,articles of manufacture, and improvements as described above forphotovoltaic cells.

Without further analysis, from the foregoing others can, by applyingcurrent knowledge, readily adapt the present invention for variousapplications. Such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

1. A solar cell assembly including a terminal of first polarity and aterminal of second polarity comprising: a first discrete solar cellincluding a top contact connected to the terminal of first polarity, anda bottom contact; a second discrete solar cell disposed spaced apartfrom, mounted adjacent to, and with respect to the incoming illuminationparallel to the first solar cell, the second discrete solar cellincluding a tandem vertical stack of at least a first upper solarsubcell, a second solar subcell, and a third bottom solar subcell;wherein the first upper subcell of the second solar cell having a topcontact connected to the terminal of first polarity; wherein the thirdbottom subcell of the second solar cell having a top contact and abottom contact, the bottom contact being connected to the terminal ofsecond polarity; and wherein the bottom contact of the first solar cellis connected in a series electrical circuit with the top contact of thethird solar subcell of the second discrete solar cell so that theelectrical interconnection of the first and second discrete solar cellsforms the solar cell assembly with said terminals of first polarity andsecond polarity.
 2. A solar cell assembly as defined in claim 1, whereinthe upper first solar subcell of the second solar cell is composed ofaluminium indium gallium phosphide (AlInGaP); the second solar subcellis disposed adjacent to and lattice matched to said upper first solarsubcell, and is composed of aluminum gallium arsenide (AlGaAs) orgallium arsenide (GaAs), and the third bottom solar subcell is latticematched to said second solar subcell and is composed of germanium (Ge).3. An assembly as defined in claim 1, wherein the second solar cellfurther comprises a first highly doped lateral conduction layer disposedadjacent to and beneath the second solar subcell, and a blocking p-ndiode or insulating layer disposed adjacent to and beneath the firsthighly doped lateral conduction layer, and a second highly doped lateralconduction layer disposed adjacent to and beneath the blocking p-n diodeor insulating layer.
 4. A solar cell assembly as defined in claim 1,wherein the short circuit density (J_(sc)) of the third bottom subcellis at least twice that of the first and second solar subcells.
 5. Asolar cell assembly as defined in claim 1, wherein the short circuitcurrent density (J_(sc)) of the first and second solar subcells are eachapproximately 17 mA/cm², and the short circuit current density (J_(sc))of the third bottom subcell is approximately 34 mA/cm².
 6. A solar cellassembly as defined in claim 2, further comprising a fourth solarsubcell composed of gallium arsenide (GaAs) disposed adjacent to andbeneath the second solar subcell, and above the third bottom solarsubcell.
 7. A solar cell assembly as defined in claim 6, wherein theshort circuit current density (J_(sc)) of the first, second and thirdsolar subcells are each approximately 11 mA/cm².
 8. A solar cellassembly as defined in claim 7, wherein the short circuit currentdensity (J_(sc)) of the third bottom solar subcell is approximately 22.6mA/cm².
 9. A solar cell assembly as defined in claim 6, wherein at leastthe base of at least one of the first, second or third solar subcellshas a graded doping.
 10. A solar cell assembly as defined in claim 3,wherein the first solar cell is a multijunction solar cell having atandem vertical stack of at least a first upper solar subcell and abottom solar subcell.
 11. A solar cell assembly as defined in claim 1,wherein the solar cell assembly comprises at least a series electricallyconnected four junction solar cell, including one subcell from the firstsolar cell and at least three subcells from the second solar cell
 12. Asolar cell assembly as defined in claim 1, further comprising aconductive interconnect extending between the bottom contact of thefirst solar cell to the top contact of the third bottom solar subcell ofthe second solar cell.
 13. A solar cell assembly as defined in claim 10,wherein the bottom contact of the third bottom solar subcell of thefirst solar cell has a first metal contact to the top surface thereof,and a second metal contact to the bottom contact thereof, and whereinthe first metal contact is electrically connected to the first highlydoped lateral conduction layer of the second solar cell.
 14. A solarcell assembly as defined in claim 10, wherein the second metal contactto the bottom contact of the bottom solar subcell of the first solarcell is electrically connected to the second highly doped lateralconduction layer of the second solar cell.
 15. A solar cell assembly asdefined in claim 14, further comprising a conductive interconnectextending between the second metal contact on the bottom contact of thebottom solar subcell of the first solar cell and the second highly dopedlateral conduction layer of the second solar subcell.
 16. A solar cellassembly as defined in claim 3, wherein the first solar cell is amultijunction solar cell having a tandem vertical stack of at least afirst upper solar subcell, a second solar subcell, and a third bottomsolar subcell.
 17. A solar cell assembly as defined in claim 16, whereinthe solar cell assembly comprises a series electrically connected fourjunction solar cell, including three subcells from the first solar celland the third bottom solar subcell from the second solar cell.
 18. Asolar cell assembly as defined in claim 1, wherein the first solar celland the second solar cell are composed of an identical sequence ofsemiconductor layers.
 19. A solar cell assembly including a terminal offirst polarity and a terminal of second polarity comprising: a firstdiscrete solar cell including a top contact connected to the terminal offirst polarity, and a bottom contact; a second discrete solar celldisposed spaced apart from, mounted adjacent to, and with respect to theincoming illumination, parallel to the first solar cell, including atandem vertical stack of at least a first upper solar subcell, a secondsolar subcell, and a bottom solar subcell, the first upper subcell ofthe second solar cell having a top contact connected to the terminal offirst polarity, the bottom subcell of the second solar cell having a topcontact, and a bottom contact connected to the terminal of secondpolarity; and an interconnect arrangement electrically connecting thefirst and second solar cells such that the bottom contact of the secondsolar subcell of the second solar cell is electrically connected to thetop contact of the bottom subcell of the first solar cell so that thecurrent generated in the first and second solar subcells of the secondsolar cell flows into and through the bottom subcell of the first solarcell, and the bottom contact of the bottom solar subcell of the firstsolar cell is connected to the top contact of the bottom solar subcellof the second solar cell that the current then flows into the bottomsubcell of the second solar cell, so that the cumulative currentgenerated in both (i) the first, second and bottom solar subcells of thefirst solar cell, and in (ii) the first and second subcells of thesecond solar cell, flows into and through the bottom subcell of thesecond solar cell, and thereafter into the bottom contact of the bottomsubcell of the second solar cell.
 20. A method of forming a solar cellassembly including a terminal of first polarity and a terminal of secondpolarity comprising: forming a first discrete solar cell including a topcontact connected to the terminal of first polarity, and a bottomcontact; forming a second discrete solar cell disposed spaced apartfrom, mounted adjacent to, and with respect to the incomingillumination, parallel to the first solar cell including a tandemvertical stack of at least a first upper solar subcell, a second solarsubcell, and a third bottom solar subcell, the first upper subcell ofthe second solar cell having a top contact connected to the terminal offirst polarity, the third bottom subcell of the second solar cell havinga top contact and a bottom contact connected to the terminal of secondpolarity; connecting the bottom contact of the first solar cell in aseries electrical circuit with the third subcell of the second solarcell so that the interconnection of the first and second solar cellsforms a solar cell assembly.